Lubricants (also referred to as lubricant compositions, lubricating oils or lubricating oil compositions), such as those used in a variety of machinery, are susceptible to oxidative deterioration during storage, transportation and usage, particularly when such lubricants are exposed to high temperatures and iron catalytic environments, which greatly promote their oxidation. This oxidation, if not controlled, contributes to the formation of corrosive acidic products, sludge, varnishes, resins and other oil-insoluble products, and may lead to a loss of designated physical and tribological properties of the lubricants. These oxidation products may lead to the formation of harmful deposits on critical engine parts, such as the pistons, piston liners, valves and valve lifters. It is therefore a common practice to include deposit-control and antioxidant additives in lubricants to prevent, at least to some extent, oxidation so as to extend their useful life.
Lubricants containing various secondary diarylamines as antioxidants are known in the art. The use of para-phenylenediamines is also known. Para-phenylene-diamines have more commonly been employed as motor fuel stabilizers and antiozonants and antioxidants for rubber.
U.S. Pat. No. 5,232,614 describes substituted para-phenylenediamines as effective antioxidants capable of protecting crankcase lubricating oils from thickening and sludge formation after prolonged exposure to oxygen at elevated temperature.
US-A-2006/0052260 describes para-phenylenediamine hydrazide compounds as antioxidants, particularly in lubricating oil compositions, including, in Example 1, the production of a Michael adduct of a phenylenediamine and butyl acrylate as an intermediate to make such a hydrazide.
US-A-2007/0006855 describes addition of a phenylenediamine compound to ameliorate soot-induced kinematic viscosity increase of lubricating oil compositions for diesel engines, particularly heavy duty diesel engines equipped with EGR systems. Also, it lists the following further references relating to phenylenediamines: U.S. Pat. No. 5,207,939, U.S. Pat. No. 5,213,699; and U.S. Pat. No. 5,298,662.
Phenylenediamines were known to act effectively as antioxidants; however they were found to be disadvantageous commercially because their presence, when used in amounts conventionally used to provide antioxidancy, displayed adverse effects on piston deposit and varnish control, and also displayed aggressiveness toward fluoroelastomeric engine seal materials. These adverse effects are particularly apparent with phenylenediamine compounds having higher nitrogen contents (compounds having relatively small hydrocarbyl substituents).
A problem in the prior art is to provide phenylenediamines that meet the above disadvantages and also to provide phenylenediamines that mitigate oxidative degradation in greases, fuels and functional fluids.